WO2006008336A1

WO2006008336A1 - Curtain coater unit and curtain coating method

Info

Publication number: WO2006008336A1
Application number: PCT/FI2005/000332
Authority: WO
Inventors: Heikki Vatanen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2004-07-20
Filing date: 2005-07-18
Publication date: 2006-01-26
Anticipated expiration: 2007-01-20
Also published as: EP1769121A4; JP4477063B2; FI20041003A0; JP2008506525A; FI116738B; EP1769121A1

Abstract

The invention relates to a curtain-coating unit (1) for coating a material web (6) arranged to move with a coating agent, which curtain-coating unit (1) includes feed elements (2) arranged above the web (6), for feeding the coating agent to the surface of the web (6) as a curtain-like flow (10). In addition, the coating unit (1) includes elements (7), by means of which the of the coating-agent feed elements (2) can be rotated around an axis (12) in the cross direction of the web.

Description

Curtain Coater Unit and Curtain Coating Method

The present invention relates to a curtain-coating unit, according to the preamble of Claim 1 , for coating a material web, which is arranged to move, with a coating agent. In a curtain-coating unit is this kind, the coating agent is fed to the surface of the material web in a curtain-like flow.

The invention also relates to a curtain-coating method for coating a moving material web.

In curtain coating, coating agent is poured as a freely falling curtain on the surface of the moving web. A curtain-coating unit includes an application beam arranged above the web, in which there is a narrow nozzle gap or a sloping feed plane fitted over the width of the web, from which the coating agent is fed to the surface of the web as a continuous curtain over the width of the web. The web, which moves at a speed many times greater than that of the downward-flowing coating-agent curtain, sweeps the coating-agent curtain into a thin film at the point of contact. Thus the final coating-agent layer is formed on the surface of the web. Using a curtain coater, the web can be feed simultaneously with several coating layers on top of each other, if the application beam is equipped with parallel nozzle gaps, from which the coating agent is fed to a sloping feed plane, so that the flows of coating agent fed from the different nozzle gaps come on top of each other before they flow onto the web.

On the surface of the material web being coated, there is a layer of air, which moves with the web to the point of contact of the coating-agent curtain with the web, i.e. the point at which the coating-agent layer falls onto the web. This boundary-layer air disturbs the application of the coating agent to the web, on account of which it is removed using boundary-layer air removal means. The boundary-layer air removal means are situated, in the direction of travel of the web, before the point at which the coating-agent curtain falls onto the web. The boundary-layer air removal means typically consist of a suction surface arranged against the web, through which the air layer on the surface of the material web is sucked off, as well as an air doctor arranged between the suction surface and the point of contact of the coating agent.

Curtain coating offers several advantages over traditional blade coating methods. In curtain coating no rapidly wearing parts, such as doctor blades or rods, are used. In addition, curtain coating places less strain on the web, thus reducing the number of web breaks occurring at the coating station. In curtain coating, the penetration of the web by the coating agent is small, which permits good coating coverage. The small coating agent penetration also reduces wetting of the base web during the coating process and thus improves the strength of the web as coating proceeds. In addition, with the aid of curtain coating it is possible to achieve a situation, in which the feed of the coating agent directly determines the amount of coating, without a return or bypass circulations, which simplifies the construction of the coating station and facilitates its operation.

When curtain coating is commenced, the amount of coating agent being fed from the application beam may vary greatly. Thus areas form on the web, in which there is a large amount of coating agent. The dryers after coating may not then be able to dry the coating layer sufficiently, in which case wet coating agent will adhere to the structures following the coater. Therefore, when coating starts, the coating agent is prevented from reaching the surface of the web, by bringing a so-called starting trough, into which the coating agent being fed from the application beam can run, between the application beam and the web. The coating agent is led from the application beam to the starting trough until the flow is as desired. After this, the starting trough is moved away from under the coating-agent curtain, so that the coating agent can reach the surface of the web. Alternatively, the coating can be started by moving the application beam in the direction of travel of the web away form the application point to a location over a starting trough fixed in place and the coating-agent flow is regulated as desired, after which the application beam is moved back to the coating position.

The solutions described above generally operate satisfactorily only in narrow coaters running at relatively low speeds. The problem in these is the large number of moving parts and, in wide coaters, the synchronization of the movement between the ends of the application beam. In addition, the removal point of the boundary-layer air must be as close as possible to point at which the coating-agent curtain falls onto the web, so that the surface of the web will not have time to develop a new boundary layer before the application of the coating agent. In curtain coaters, in which the coating agent is poured onto the web from a sloping feed plane, the coating-agent curtain turning very usually by about 10 - 20 mm towards the arrival direction of the web, during the time that it falls between the feed location and the web. The magnitude of this turning can vary if the properties of the coating agent and/or the amount being fed are changed. This may weaken the coating result, because the coating-agent curtain may strike the boundary-layer air remover, or the distance between the boundary-layer air remover and the point of contact of the curtain may grow too large.

The invention is intended to create an improved curtain-coating unit.

The invention is based on the application beam of the curtain-coating unit being able to be tilted, i.e. rotated around an axis transverse to the web, during coating. With the aid of the rotating movement, the location, in the direction of travel of the web, of the curtain of coating agent falling from the application beam can be altered during coating, for example, when coating commences.

More specifically, the curtain-coating unit according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The curtain-coating method according to the invention is, in turn, characterized by what is stated in the characterizing portion of Claim 8.

Considerable advantages are gained with the aid of the invention.

The position of the coating curtain in the direction of travel of the web can be altered by rotating the application beam around an axis in the cross direction of the web. Due to this, the solution according to the invention requires few moving parts and its construction is simple and reliable.

In one preferred embodiment of the invention, in the starting situation of the coating, the application beam is rotated to a starting position, in which the coating agent flows from the application beam to a starting trough. Once the properties of the coating-agent curtain are as desired, the application beam is rotated to the coating position, in which the coating-agent curtain can fall onto the web. Due to the rotating movement of the application beam, the movement between the ends of the beam is better synchronized.

In a second preferred embodiment of the invention, the distance of the point of contact of the coating-agent curtain from the boundary-layer air remover can be measured and adjusted while the curtain coater is running. The distance of the point of contact of the coating-agent curtain from the boundary-layer air remover can thus be kept as desired in the coating starting situation and during normal running when production conditions change, due to which the structures of the boundary-layer air remover remain cleaner and the operation of the boundary-layer air removal remains optimal.

In addition, adjustment of the distance between the point of contact of the coating-agent curtain with the web and the boundary-layer air remover allows the point of contact of the coating-agent curtain of coating agent to be kept as close as possible to the doctoring point, so that a new boundary-air layer will not have time to develop before the point of contact of the coating-agent curtain. This is advantageous, particularly in coaters running at high speed, in which a new air layer can form on the surface of the web in a distance of as little as 20 mm. hi addition, the distance adjustment permits control of the air flows prevailing in the vicinity of the point of contact of the coating-agent curtain, so that they disturb application as little as possible.

In the following, the invention is examined in greater detail with the aid of the accompanying drawings.

Figure 1 shows a cross-section of one curtain coater according to the invention, in the starting position. Figure 2 shows the curtain-coating unit of Figure 1, in the coating position.

In the curtain-coating unit 1, there are elements for guiding the continuous material web 6 to be coated and for conveying it forward in the coater 1. In the embodiment according to the drawings, a hitch roll 5 and a second hitch roll 5', which support the web 6 as it travels, act as the elements. The hitch rolls 5, 5' are mounted at their ends in bearings running in bearing shells, which are in turn attached to the frame elements 3 of the coating unit 1. An application beam 2 is fitted above the hitch roll 5, to feed the coating mixture to the web 6 as a freely falling curtain 10. In the application beam 2, there is a nozzle or other feed component extending over the entire width of the web 6, from which the coating agent is fed to the web 6 over the entire width of the web, or extending essentially over the entire width of the web, as a freely falling curtain 10. The other feed component can be, for example, a sloping feed plane, from which the coating mixture falls in a curtain-like flow to the surface of the web.

In the embodiment shown in the drawings, the application beam 2 is equipped with a sloping plane 13 and three parallel feed gaps 14. Each feed gap 14 extends over the entire width of the web 6. Coating agent is fed from the feed gaps 14 to the sloping feed plane 13, from which the coating agent flows on over the lip and falls in a curtain-like flow onto the web 6 at the point of contact 9. The coating-agent flows fed from the different feed gaps 14 are on top of each other on the feed plane 13, so that the coating agent fed from the feed gap located closest to the Hp 11 is the lowest and the coating agent fed from the feed gap farthest from the lip 11 is on top. By using feed gaps 14 set on top of each other in the application beam 2, several layers of coating agent can be applied on top of each other simultaneously.

The application beam 2 is attached rigidly to the frame elements 3 of the curtain coater, for example, with the aid of bearers in the frame element. The frame element 3 is, in turn, attached rotatably to the frame 4 of the coater by hinges, with the aid of which the frame element 3 can rotate around an axis 12 at the hinge point running in the cross direction of the web. Operating devices 7 are arranged between the frame element 3 and the frame 4, with the aid of which the frame element 3 can be rotated around the axis 12. For example, hydraulic cylinders, the first end of which is hinged to the frame element 3 and the second end to the frame 4, are used as the operating devices 7. The rotational movement of the frame element 3 is limited to the desired amount by means of stops, against which the operating devices 7 press the frame element 3 at the extreme positions of the rotational movement. The rotational movement is preferably limited in such a way that the frame element 3 and the application beam 2 can rotate around the axis 12 by at most 45° counterclockwise, preferably by at most 15°. Typically, the frame element 3 and the application beam 2 can be rotated around the axis 12 by about 15°. The angle of rotation can be preferably typically also 10°.

The curtain-coating unit 1 also includes a trough-like starting trough 8, to which the coating agent fed from the application beam 2 is fed when coating starts (Fig. 1). The starting trough 8 is attached to the frame element 3. From the starting trough 8, the coating agent flows on to the coating mixture circulation. The starting trough 8 is located between the application beam 2 and the web 6, at a point at which the coating-agent curtain falls from the application beam 2 when coating starts, so that the coating agent cannot reach the surface of the web 6. In the cross direction of the web 6, the starting trough 8 extends at least over the width from which the coating agent is fed from the application beam 2 to the web, so that in the starting situation all of the coating mixture coming from the application beam 2 falls into the starting trough 8. In the embodiment shown in the drawings, the starting trough 8 is located above the hitch roll 5. When the frame element 2 is rotated around the axis 12, the starting trough 8 also rotates.

The curtain-coating unit 1 also includes boundary-layer air removal means 15, which are used to remove the boundary-layer air on the surface of the web 6 before the point of contact 9 of the coating-agent curtain 10 with the web. The boundary-layer air is a layer of air that has formed on the surface of the web 6, which moves together with the advancing web 6. The boundary-layer air removal means 15 are attached to the frame element 3 and are located above the web 6. The boundary-layer air removal means 15 include a suction surface arranged facing the upper surface of the web 6, in which there is, for example, a nozzle or openings, through which the boundary-layer air is sucked off the surface of the web 6. The suction surface extends over the entire width of the web.

When the frame element 3 is rotated around the axis 12, the boundary-layer air removal means 15 also rotate.

In addition, the boundary-layer air removal means 15 include a doctor fitted against the web 6 and extending over the entire width of the web, for example, a blade or rod doctor, by means of which the boundary-layer air is also removed from the surface of the web 6. The doctor is located after the suction surface in the direction of travel of the web 6, but nevertheless before the point of contact 9 of the coating-agent curtain 10 with the web. Between the front edge of the pouring trough 8 and the rear edge of the boundary- layer air removal means 15 there is a gap, through which the coating-agent curtain 10 reaches the surface of the web 6. The width d of the gap is preferably 30 - 80 mm. The direction of travel of the web 6 is shown in the drawings by an arrow. It is also possible to remove the boundary-layer air without contact.

The curtain-coating unit 1 also includes guard plate 16, which is fitted between the application beam 2 and the boundary-layer air removal means 15. A closed air space is formed between the guard plate 16 and the coating-agent curtain 10, into which stabilization air can be blown during coating, if necessary. The behaviour of the curtain 10 in the area of the point of contact 9 is controlled with the aid of the stabilization air.

The stabilization air is blown into the area of the point of contact 9 of the coating-agent curtain 10 using stabilization-air blowing means and is removed from the area of the point of contact by stabilization-air removal means, m the curtain-coating unit according to the invention the stabilization air blowing and removal means disclosed in patent application FI 20035108, for example, can be used.

In addition, the curtain-coating unit 1 includes a measuring device 17, by means of which the distance between the point of contact 9 of the coating-mixture curtain 10 with the web and the boundary-layer air removal means 15 can be measured while the coating unit 1 is running. The measuring device 17 is, for example, a laser measuring device, which includes means for sending a laser pulse to the curtain 10 of coating mixture and means for receiving the reflected laser pulse. The distance of the coating-agent curtain 10 from the measuring device 17 can be determined on the basis of the time taken for the laser pulse to travel the distance between the transmitter and the receiver. Further, on this basis it is possible to calculate the distance between the boundary-layer air removal means 15 and the point of contact 9. The measuring device 17 is located, for example, beneath the starting trough 8.

When coating starts, the frame element 3 and the application beam 2 attached to it are first rotated around the axis 12 by the operating devices 7 to the starting position according to Figure 1. In the starting position, the lip 11, or other component of the application beam 2, from which the coating-agent curtain 10 begins to flow freely downwards towards the web 6, is above the starting trough 8. After this, the feed of the coating agent to the sloping plane 13 from the feed gaps 14 is commenced. The coating agent fed from the application beam 2 is allowed to flow to the starting trough 8 until the properties of the coating-agent curtain, such as the feed amount, have been adjusted as desired.

After this, the feed of the coating agent from the application beam 2 is continued and the frame element 3 and the application beam 2 are rotated around the axis 12 by the operating devices 7 to the coating position according to Figure 2. In the coating position, the coating-agent curtain 10 fed from the application beam 2 falls through the gap between the starting trough 8 and the boundary-layer air removal means 15 to the point of contact 9 of the surface of the moving web 6. The web 6, which moves at a speed many times that of the downwardly flowing coating-agent curtain 10, sweeps the coating-agent curtain at the point of contact 9 into a thin film, so that the final layer of coating agent is formed on the surface of the web 6. The boundary-layer air on the surface of the web 6 is removed during application by the boundary-layer air removal means 15.

The distance of the coating-agent curtain 10 from the boundary-layer air removal means 15 is measured using the measuring device 17 during the operation of the coating unit 1. If the distance measured deviates from the desired distance, the application beam 2 and the frame element 3 are rotated around the axis 12 by the operating devices 7, to bring the distance back to that desired. The distance between the point of contact 9 of the coating-agent curtain 10 and the boundary-layer air removal devices 15 is held during operation of the curtain coater typically to 5 - 75 mm, and preferably to 15 - 20 mm.

Embodiments of the invention, differing from those disclosed above, can also be envisaged.

In the embodiments according to the drawings, both the frame element 3 and the application beam 2 attached rigidly to it are rotated around an axis 2 running in the cross direction of the web, in order to move the point of contact of the coating-agent curtain 10 with the web 6 and in the starting situation. The rotational movement of the application beam 2 can alternatively be implemented by using hinges to attach the application beam 2 to the frame element 3. In that case, only the application beam 2 can be rotated at the hinge points around an axis in the cross direction of the web, while the frame element 3 remains stationary.

Claims

Claims:

1. Curtain-coating unit (1) for coating a material web (6) arranged to move with a coating agent, which curtain-coating unit (1) includes feed elements (2) arranged above the web (6), for feeding the coating agent to the surface of the web (6) as a curtain-like flow (10), characterized in that the feed elements (2) of the coating agent can be rotated around an axis (12) in the cross direction of the web (6), and that the coating unit (1) includes means (7) for rotating the feed elements (2) around the axis (12).

2. Coating unit according to Claim 1, characterized in that the feed elements (2) of the coating agent are attached to a frame element (2), which can be rotated around the axis (12).

3. Coating unit according to Claim 1 or 2, characterized by a starting trough (8) for preventing the flow of coating agent fed from the coating-agent feed element (2) from reaching the web (6) during the starting of coating.

4. Coating unit according to Claim 3, characterized in that the coating-agent feed elements (2) can be rotated around the axis (12) between a starting position and a coating position, in which starting position the coating agent can flow from the feed elements (2) to the starting trough (8) and in which coating position the coating agent can flow from the feed elements (2) to the surface of the web (6).

5. Coating unit according to any of the above Claims, characterized by boundary-layer air removal means (15) for removing the boundary-layer air on the surface of the web

(6), which boundary-layer air removal means (15) are arranged, in the direction of travel of the web (6), before the point of contact (9) of the coating-agent curtain (10) with the web.

6. Coating unit according to any of the above Claims, characterized by a measuring device (17) for measuring the distance between the coating-agent curtain (10) and the boundary-layer air removal means (15).

7. Coating unit according to any of the above Claims, characterized in that the coating- agent feed elements (2) can be rotated around the axis (12) by at most 45°, and preferably by about 15°.

8. Coating unit according to any of the above Claims, characterized in that the coating- agent feed elements (2) can be rotated around the axis (12) by at most 45°, and preferably by about 10°.

9. Curtain-coating method for coating a material web (6) arranged to move with a coating agent, in which method the coating agent is fed from coating-agent feed elements (2), arranged above the web (6), towards the web (6) as a curtain-like flow (10), characterized in that the coating-agent feed elements (2) can be rotated around an axis (12) in the cross direction of the web (6), in order to alter the location of the coating-agent curtain (10).

10. Method according to Claim 9, characterized in that the coating-agent feed elements (2) are rotated around the axis (12) between a starting position and a coating position, in which starting position the coating agent can flow from the feed elements (2) to a starting trough (8) and in which coating position the coating agent can flow from the feed elements (2) to the surface of the web (6).

11. Method according to Claim 9 or 10, characterized in that the distance between the point of contact (9) of the coating-agent curtain (10) with the web and the boundary- layer air removal elements (15) is measured and the distance is altered as desired by rotating the coating-agent feed elements (2) around an axis (12) running in the cross direction of the web (6).